Autothrottle systems are utilized on many jet aircraft, particularly commercial jet aircraft, to provide automatic control of engine thrust. Thrust is increased by advancing the throttle and decreased by retarding the throttle. The flow of fuel to the engines of the aircraft is metered by the engine control in response to the throttle inputs.
One goal of an aircraft autothrottle system is to change thrust as rapidly as possible during transients, i.e., changes in speed. A difficulty arises when throttle changes are made too rapidly. The engine control will limit the rate of fuel flow change to maintain stable engine operation during the transient. When this occurs the engine control(s) operate at saturation, e.g., they operate at their fuel flow to burner pressure ratio limit or their maximum rate of change of rotor speed limit. When operating at these limits, throttle lever angle leads engine thrust. This can result in the throttle lever angle oveshooting the target lever angle position and/or engine thrust overshooting the target thrust level. In either case, unnecessary throttle adjustments are required to achieve the target condition.
The majority of prior attempts to improve autothrottle-engine response characteristics have centered on the autothrottle side of the overall system. Sophisticated autothrottle computers have been developed. Such computers are programmed with information regarding the aircraft engine's response characteristics and monitor several engine parameters in an attempt to properly schedule throttle rate during transients. Such systems have several disadvantages. Significant amounts of computer storage space are required to store engine response characteristics. Further, the stored information must be updated for each new engine type. Also, because the engine data is for a nominal engine, it may not always accurately reflect the response of a particular engine. In addition, prior autothrottle-engine response systems have required either several dedicated wires or significant space on a data bus in order to receive the required engine parameter signals. Overall, prior systems designed to improve the autothrottle-engine response characteristics have involved a great deal of engineering development work that must be repeated for each new engine to be certified. Much of this work represents a duplicity of effort as the propulsion staffs of both the engine manufacturer and the aircraft manufacturer develop a great deal of engine transient response characteristic information for each type of engine utilized on commercial and other aircraft. Further, even these sophisticated attempts to improve autothrottle-engine response have not been entirely satisfactory and have not achieved the optimum response.